Pressure-sensitive adhesive tape



Sept. ll, 1951 E. o. JOESTING 2,567,671

PRESSURE-SENSITIVE ADHESIVE TAPE Filed oct. 1o, 1949 Patented Sept. 11, 1951 nessuna-SENSITIVE ADHESIVE 'rara Edwin ofrmung, st. raul, Minn., mmm u Minnesota -Mining and Manufacturing Company,- St. Paul, Minn., a corporation of Dela- Application October 10. 19.49, Serial No. 120,491 2 Claims. (Cl. 117-122) This invention relates to normally tacky and pressure-sensitive adhesive tapes, and particularly to transparent tapes having age-resistant transparent adhesives.

Transparent tapes, consisting essentially of transparent normally tacky and pressure-sensitive adhesive coatings firmly bonded to transparent flexible backing members such as cellophane or cellulose acetate films, are widely employed for sealing packages, hanging posters, mending books, and other applications. The adhesive layer desirably consists of natural or synthetic rubber and a resinous tackifi'er. These raw materials provide generally acceptable adhesive compositions, and are readily available at reasonable cost. Deterioration of such adhesives under exposure to sunlight has long been a problem, the eifect of sunlight being to soften and liquify the adhesive.

One test which the tapeI is required to pass before it can be accepted for certain applications is a sun-lamp exposure test. The test section of tape is rolled into full contact with a sheet of sulphate paper of letterhead grade and is then exposed continuously for 30 hours to standard illumination, directed vertically at the exposed transparent backing, from a General Electric S-l sunlamp at a distance of 30 inches and under conditions of 120 F. Under these conditions it has been found that all available transparent adhesives based on natural rubber soften or partially llquify and penetrate through the test paper.

It is an object of the present invention to produce a transparent normally tacky and pressure-sensitive adhesive from a combined natural and synthetic rubber base and having improved properties, particularly with respect to resistance to sunlight. Another object is the production of a normally tacky and pressure-sensitive adhesive tape and particularly a transparent tape which under the test conditions above defined properties over similar combinations employing.

rosin or the like but the adhesives nevertheless fail under the sun-lamp exposure test and tapes coated with these adhesive masses have not been satisfactory for a number of applications.

When GR-S synthetic rubber is substituted for the natural rubber of the above adhesives. th'e resulting blend is found to be completely nontacky and cloudy or opaque in appearance.

Surprisingly, I have now discovered that GR-S rubber may be blended with natural rubber to will not result in penetration of the test paper by the adhesive. Other objects will be apparent in view of the following description and the appended claims.

Polymerized terpene resins, particularly polymerized beta-pinene resins, have previously been suggested as tackiers for natural rubber land for certain synthetic rubbery polymers-in the, production of normally tackyand pressure-sensi- A typical example of 1 suchA la vresin Ais the pure hydrocarbonA therunoplastic',''` terpene resin melting at 85 C., having` a zero` `tive adhesives.

acid number and known as Piccolite S-85. The

produce a rubbery base with which polymerized terpene resins of zero acid number and having melt 'g points of at least abot 80 C. are fully com tible and in which the resin exhibits fully as eat tackifying power as in the absence vof the GR-S rubber. The resulting adhesive is found to be highly resistant to degradation by sunlight and effectively meets the sunlight exposure test above described. These novel adhesives permit of making transparent pressuresensitive adhesive tapes of high quality from all standpoints.

The following example represents a typical formulation of many novel adhesive compositions.

Example: l Parts by weight Latex crepe natural rubber Rubbery butadiene-styrene copolymer (e. g., GR-S X-274) Anti-oxidant (e. g., alkylated polyhydroxy phenol) Polymerized terpene resin (e. g.. Piccolite S-) Heptane 600 The several solid components may be dissolved blended together by milling on a rubber mill and are then dissolved, e. g. in a paddle-type mixer.

The adhesive composition is coated in a thin uniform layer on the previously primed surface ofv cellophane and the solvent removed by evaporation to produce a normally tacky and pressure-sensitive transparent adhesive tape. After testing for 30 hours in the sunlight exposure test, such tape produces no discoloration of the back surface of the test paper and is therefore judged satisfactory.

The copolymer of the above formula represented a ratio of 72 parts butadiene and 28 parts styrene. This polymer as produced commercially is known as GEL-S X-274 and contains a small proportion, viz. 1.25%, of a mixture of mono and di-hepto diphenyl amines as an anti-oxidant. Other rubbery GR-S type polymers have been employed with substantially equivalent results. For example a copolymer of 50 parts of butadiene and 50 parts oi styrene has replaced the above poymer in the formula given.

Likewise other terpene resins may replace the specific resin identied in the formula. 'In particular polymerized beta-pinene resins of somewhat higher melting points have been found to provide excellent properties. For example Piccolite S-i25 melting at 125 C. has given good results and the adhesive is somewhat higher in melting point than the one shown hereinabove.

The formula of the above example represents preferred proportions and preferred component ingredients. it is apparent from the foregoing paragraphs that other equivalent ingredients may be substituted for those there specied. Likewise some substitutions in proportions are found to provide excellent adhesive characteristics and are here contemplated. However too high a proportion or the resinous component, e. g. the Piecolite S-85 is found to produce unclesirably high adhesion to the tape backing when the tape is wound in roll form and frequently 'results in tearing and breaking of the tape during unwinding. Too low a proportion of the tackiier resin results in an undesirably low adhesion value and the tape is'unsatisfactory for its intended purpose. With extremely small percentages of the GRHS rubber the adhesive Kshows no apparent improvement over the two-component system of natural rubber and resin which, as previously noted hereinabove, was ineffective in meeting the sunlight exposure test and the various practical applications on which such test is based. On the other hand, too high a proportion of GR-S rubber has been observed to result in reduced tackiness of the adhesive and in varying degrees of opacity.

.--L\'1he single igure of the drawing represents Vthe form of a roll o1' tape, to provide good unwind characteristics. The shaded area of the drawing represents this useful range which will be observed to liek within the approximate proportions oi 25 to 75 parts natural rubber, 25 to 75 parts GR-S synthetic rubber and from about 40 to about parts of polymerized terpene resin based on parts oi' combined rubbers. Adhesive compositions falling within this area have been found when coated on suitable transparent backings to provide clear, transparent, normally tacky and pressure-sensitive adhesive tapes eminently suitable for applications involving attractive appearance and good adhesive holding power under exposure to sunlight.

Having disclosed various embodiments of my invention for purposes of illustration rather than limitation, what I claim is as follows:

1. Normally tacky and pressure-sensitive adhesive tape comprising a flexible backing and a irmly adherently bonded coating thereon of an adhesive consisting essentially of a blend of about 25-75 parts of natural rubber, correspondingly about 'l5-25 parts of butadienestyrene synthetic rubber, and, based on 100 parts of the combined rubbers, about 40-90 parts of a thermoplastic terpene resin of zero acid number and having a melting point of at least about 80 C., the proportions of the above three components being substantially within the range indicated by the shaded area of the drawing.

2. Transparent, normally tacky and pressuresensitive adhesive tape comprising a transparent lm backing and a rmly adherently bonded REFERENCES CITED The following references are of record in the le o this patent:

UNITED STATES PATENTS Number Name Date 2,411,905 Stephens Dec. 3, i946 2,458,166 Homeyer Jan. 4, 1949 2,459,891 Nelson et al. Jan. 25, 1949 2,468,482 Campbell Apr. 26, 1949 

2. NORMALLY TACKY AND PRESSURE-SENSITIVE ADHESIVE TAPE COMPRISING A FLEXIBLE BACKING AND A FIRMLY ADHERENTLY BONDED COATING THEREON OF AN ADHESIVE CONSISTING ESSENTIALLY OF A BLEND OF ABOUT 25-75 PARTS OF NATURAL RUBBER, CORRESPONDINGLY ABOUT 75-25 PARTS OF BUTADIENESTYRENE SYNTHETIC RUBBER, AND, BASED ON 100 PARTS OF THE COMBINED RUBBERS, ABOUT 40-90 PARTS OF A THERMOPLASTIC TERPENE RESIN OF ZERO ACID NUMBER AND HAVING A MELTING POINT AT LEAST ABOUT 80* C., THE PROPORTIONS OF THE ABOVE THREE COMPONENTS BEING SUBSTANTIALLY WITHIN THE RANGE INDICATED BY THE SHADED AREA OF THE DRAWING. 